1. Field of the Invention
The invention relates to an amplifier that comprises an electronic tube with an axial electron beam and voltage sources; the electron tube comprises at least two collectors, each collector being connected to a voltage source.
The field of the invention is that of the power amplification of UHF signals, in particular the amplification of analog and digital television signals from terrestrial transmitters, by means of vacuum tubes comprising an axial electron beam.
A vacuum tube uses the principle of the interaction between an electron beam and an electromagnetic wave in order to transfer to the wave a part of the power contained in the electron beam, so as to obtain at the output of the tube a wave of higher power than that of the wave injected at the input of the tube.
Several categories of vacuum tube exist. In traveling wave tubes, or TWTs, and klystrons, the electron beam is modulated in velocity. In inductive output tubes, or IOTs, the electron beam is modulated in density.
The principle of this interaction applied to an IOT tube, shown in FIG. 1, will be described.
An IOT tube comprises an elongated vacuum envelope composed, in part, of several insulation ceramics 23a, 23b, 23c, 23d, with an electron gun at a first end emitting an electron beam 12 and, at a second end, a first collector 14a and a second collector 14b. The electron gun comprises a cathode 16 that emits the electrons and a grid 18 that controls the flux of electrons as a function of the voltage applied to the grid 18.
The signal to be amplified, of power Pe, is injected between the cathode 16 and the grid 18 and thus modulates the voltage on the grid. The electron beam 12 crossing the grid is then modulated in density by the grid and the electrons are emitted in the form of packets; the time interval between two packets is equal to the period of the signal. The beam 12, shown in the form of packets of electrons, is substantially cylindrical over almost the whole length of the tube between the cathode 16 and the collectors 14a and 14b. This cylindrical form is obtained due, on the one hand, to the shape of the cathode 16, of the anode 24 and of the grid 18 and, on the other, due to an axial magnetic field that keeps the electrons close to the axis 10 of the tube.
A packet of electrons coming from the grid 18 is accelerated before entering a drift tube 20 then into an interaction gap situated between two interaction electrodes 22a, 22b; this interaction gap is connected to a primary output cavity 26 in which the interaction gap generates an electric field. When another packet of electrons reaches this interaction gap, the packet of electrons meets this electric field which slows it down. During this slowing, the kinetic energy of the electrons is converted into electromagnetic or microwave energy, in other words into output power Ps that is directed toward the application, for example via a coaxial feed through an insulator 25.
For a television signal, the power efficiency is generally of the order of 20 to 40%: it represents the part of the power of the electron beam converted into power in the amplified signal.
The remaining power in the electron beam, after its passage through the primary output cavity 26, is subsequently dissipated in the collector. The electrons then bombard the walls of the collector and transform their kinetic energy into heat.
The electrons that reach the collector have very variable energy levels. In order to improve the power efficiency of these tubes, the collector is divided into two collectors 14a and 14b that are electrically isolated from one another; each of these collectors has a potential applied to it that corresponds to one of the energy levels of the electrons. The second collector 14b has a lower potential applied to it than the first collector 14a, with respect to the cathode 16, in order to slow down the electrons that impact the second collector 14b and thus reduce the energy lost in the form of heat. An efficiency up to three times higher than the efficiency of a conventional tube can thus be obtained.
2. Description of the Related Art
Tubes comprising more than two collectors have already been produced as have tubes comprising a repelling electrode at the back end of the collector, this electrode generally being connected to the cathode.
The collector 14b is connected to the positive pole 28 of a DC voltage source 30, for example of 26 kV. The negative pole 32 of the DC voltage source 30 is connected to the cathode 16.
The collector 14a is connected to the positive pole 34 of a DC voltage source 36, for example of 34 kV. The negative pole of the DC voltage source 36, common to that of the voltage source 30, is also connected to the cathode 16. The two sources therefore have a common point 32 situated at the cathode. The output cavity 26 is also connected to the positive pole 34 of the DC voltage source 36, if required via a measurement shunt between ground 17 and the casing of the tube that comprises the anode 24 and the two interaction electrodes 22a, 22b; this measurement shunt allows the current intercepted by the body to be measured. The output cavity 26 is connected to ground.
A current I of a few Amps (for example 2.5 A) coming from the cathode 16 is obtained in the electron beam 12.
According to this configuration, two voltage sources 30 and 36 of over 26 kV are used, which are bulky and costly given the value of their voltage, the distances necessary for their isolation and also their respective powers.
Furthermore, the potential difference between the two collectors 14a and 14b, also called the depression voltage, must not exceed 12 kV in order not to damage the tube, especially the ceramic 23a situated between the two collectors. However, in this type of voltage source, when one of the two collectors 14a, 14b is rapidly interrupted by the action of the safety systems or turns on faster than the other during power-up, a differential voltage that can reach 34 kV is generated between the two collectors 14a and 14b causing, possibly irreversible, damage. The necessity for synchronizing these voltage sources makes this configuration very constraining.